Adapt Duo Chat Slash commands for multi-threaded

This is a draft MR to update the slash command management in duo-chat multi-threaded

• it will display /new in the slash command picker
• it will create a new thread on /new /reset /help

this will resolve: #470818 (closed) & #523267 (closed) & #517290 (closed)

How to reproduce:

1. Set up DuoChat locally
2. rails c -> Feature.disable(:duo_chat_multi_thread)
3. make sure /clear & /reset are shown in the slash command picker and working as expected
4. rails c -> Feature.enable(:duo_chat_multi_thread)
5. make sure /new is shown in the slash command picker and working as expected, make sure when entered /clear or /reset a new thread is triggered.

Demo
2025-03-10_14-41-55